Hitherto, casting has been used as a technology of forming an aluminum alloy and the like. As an example of casting methods, there has been used a die casting method, which involves injecting molten metal into a die under pressure so as to obtain a product having a predetermined shape. However, the molten metal is mainly used in the die casting method, thereby causing problems such as short lifetime of the die, and unsatisfactory quality of a product caused by generation of a shrinkage cavity or the like.
Accordingly, in recent years, as the die casting method, there has been used a casting method to be performed under high pressure using, as a metal material to be injected into the die instead of the molten metal, metal (semi-solid metal or semi-molten metal) assuming a semi-molten state in which a solid phase component and a liquid phase component coexist.
This method is distinguished from general die casting methods, and called a rheocasting method or a thixocasting method.
The rheocasting method is conducted in the following manner. Specifically, solidifying metal is forcibly stirred (or agitated) electromagnetically, mechanically, or by means of ultrasonic waves or the like, to thereby obtain semi-solid metal having a solid-liquid mixed phase in which fine spherical crystallites are dispersed homogeneously in a liquid phase. The semi-solid metal is injected under pressure into a mold of a die-cast machine, to thereby form a product by casting.
The thixocasting method is conducted in the following manner. Specifically, semi-solid metal is obtained by forcibly stirring molten metal while cooling the molten metal. Then, the semi-solid metal is temporarily cooled quickly, and then completely solidified so as to form an ingot (billet) having a bar-like shape. When manufacturing a product, a piece of a necessary amount is cut out of the billet, and then the piece is reheated so as to assume a semi-molten state (semi-solid state). Through this procedure, a product is manufactured using a die-cast machine or the like similarly to the rheocasting method.
The both methods have an advantage and a disadvantage. The both methods are common in that the semi-solid metal (hereinafter also representing the semi-molten metal) is formed under pressure in the mold.
Incidentally, when injecting the metal material into the die under pressure by the above-mentioned methods, it is necessary to set the semi-solid metal in a casting sleeve and extrude (inject) the metal into the mold by a pressure device such as a plunger. However, at a stage of inserting the semi-solid metal in the sleeve, the metal is brought into contact with the sleeve, to thereby lose its heat. Thus, a solidified layer is liable to be generated. Accordingly, an inventive way is demanded for preventing the solidified layer from being contained in a product.
Further, while filling the semi-solid metal, the sleeve or the like requires a pressure portion called a biscuit sandwiched between the plunger and a terminal end of the sleeve, a runner (sprue runner) leading the semi-solid metal into the die, and the like similarly to die casing. Further, in order to control inflow rate (reduce the inflow rate), a runner having a large cross-sectional area is required. Those portions do not form a product, thereby leading to a cause of a large amount of wasted material, reduced yield, and increased manufacturing cost.
Further, the semi-solid metal has a higher coefficient of friction with respect to the sleeve and the die than the molten metal, and hence it is necessary to increase a force of pressing the plunger as compared to a case of the molten metal. Further, it is necessary to provide a device for generating a larger force of pressing the plunger as compared to the case of the molten metal, thereby causing a problem such as increased device cost, which is a cause of increased manufacturing cost.
In view of the above-mentioned circumstances, there has been developed a forming method involving inserting the semi-solid metal (or semi-molten metal) directly into a forming die.
For example, Patent Literature 1 discloses the following technology. Specifically, semi-solid metal held in a holding vessel is inverted and placed in a recess of a lower die, and an upper die is lowered so as to compress-deform the semi-solid metal softly into a basic shape. Then, the semi-solid metal is formed into a product having a finished shape.
Further, Patent Literature 2 discloses the following method. Specifically, semi-molten metal (semi-solid metal) is charged into a cavity of a die (lower die) of a pressing machine, and an upper die is lowered. Primary forming is performed while applying pressure until a temperature of the metal in the cavity reaches a solidification finish temperature. Then, secondary forming of a product is performed by changing a shape of the cavity with a second pressure device.
Further, Patent Literature 3 discloses the following forming method. Specifically, semi-molten metal or semi-solid metal is charged into a die. First pressurizing (primary mold clamping) is performed on the die, and then second pressurizing (secondary mold clamping of forming a finished product) is performed.
Further, Patent Literature 4 discloses the following preventing method. Specifically, in order that a position of charging semi-solid metal can be corrected, the semi-solid metal is solidified so as to have a proper solid phase ratio, and thus a liquid phase component is reduced. Thus, dripping of the liquid phase component and crumble of the semi-solid metal are prevented. With this method, a satisfactory product can be obtained.
The four methods are common in that the semi-molten metal (semi-solid metal) is charged into the cavity of the die, and then pressure forming is performed.
Here, Patent Literature 1 corresponds to JP 2003-136223 A, Patent Literature 2 corresponds to JP 2007-118030 A, Patent Literature 3 corresponds to JP 2011-67838 A, and Patent Literature 4 corresponds to JP 2014-18823 A.
It is considered that, when the above-mentioned forming methods disclosed in Patent Literature 1, Patent Literature 2, Patent Literature 3, and Patent Literature 4 are used, a high-quality product having no shrinkage cavity can be manufactured at low cost using the semi-molten metal or the semi-solid metal.
Incidentally, in the method involving charging the semi-solid metal into the cavity of the die to manufacture a product under pressure, in general, in order to achieve easiness of handling of the semi-solid metal when charging the semi-solid metal into the die, and to achieve quality improvement such as reduction of a shrinkage cavity, there is used semi-solid metal that is adjusted so as to have a relatively high solid phase ratio of metal.
For example, in Patent Literature 1, the solid phase ratio of the semi-solid metal is set to 30 to 99.9%. When charging the semi-solid metal into the die, the semi-solid metal that is adjusted so as to have a predetermined solid phase ratio is contained in the vessel, and the vessel is conveyed to a position of the cavity of the die. Then, the vessel is tilted, and thus the semi-solid metal is charged (or supplied) into the cavity of the die.
However, even when the semi-solid metal is charged into the cavity of the die carefully by this inventive way, it is actually difficult to charge the semi-solid metal into the cavity of the die homogeneously and uniformly.
Patent Literature 4 describes the following matter. Specifically, when charging the semi-solid metal (molten metal) into the cavity of the die, the semi-solid metal (molten metal) falls down from a container containing molten metal (metallic container) differently each time, and hence falling positions of the semi-solid metal vary in the die. Accordingly, it is necessary to correct a charging position. Further, Patent Literature 4 describes that even correction is difficult in a case of crumbling slurry.
When the semi-solid metal (molten metal) is charged into the cavity of the die, that is, when the semi-solid metal is discharged from the metallic container, although a draft (draft angle) is formed in the metallic container and a mold lubricant is applied to the metallic container, a time period for discharging the semi-solid metal from the metallic container is not stabilized due to a wall thickness of the metallic container, variations of a molten metal injecting temperature and a molten metal injecting amount, and variations of application of the mold lubricant in a case where the semi-solid metal has a relatively high solid phase ratio, specifically, 30% to 99.9%.
Accordingly, the semi-solid metal in the metallic container is discharged in the midst of tilting the metallic container, or discharged with a certain time interval after the semi-solid metal is completely inverted. In addition, the semi-solid metal sometimes crumbles in a case where the solid phase ratio is low. There may be caused a phenomenon that the position of charging the semi-solid metal into the die is variable. Variations of the position of charging the semi-solid metal into the die cause excess and deficiency of the semi-solid metal in a portion to be filled, and actually deteriorate dimension accuracy (see Patent Literature 4).
Further, as a method of generating the semi-solid metal, the following method has been widely employed owing to its excellence in economy. Specifically, the molten metal is injected into the metallic container, and electromagnetic, mechanical, or oscillational stirring is performed while a temperature of the molten metal in the metallic container is decreased to a liquid phase temperature or less. In this manner, the semi-solid metal in which a liquid phase and a solid phase are mixed is obtained.
In this case, heat of the molten metal transfers to the metallic container from the molten metal in the metallic container, and then transfers from the metallic container to the open air. Accordingly, the temperature of the semi-solid metal contained in the metallic container is low on a side close to the metallic container in a radial direction of the metallic container, and is high on a center side of the semi-solid metal. A large amount of the liquid phase is liable to be generated at the center portion on the high-temperature side.
In a case of charging the semi-solid metal in this state, there arises a phenomenon that, when tilting the metallic container, the center portion of the semi-solid metal having a large amount of the liquid phase flows out, and then flows down first into the die. In this case, a part of the semi-solid metal having a large amount of the solid phase is charged onto a part of the semi-solid metal having a large amount of the liquid phase and having flowed down first.
When forming is performed using the material charged in this manner, the part of the semi-solid metal having a large amount of the liquid phase and having flowed in first is brought into contact with the die, and is solidified prior to the remaining part. After that, the part of the semi-solid metal having a large amount of the solid phase is formed under pressure. As a result, such a product is actually produced that the part having a large amount of the liquid phase adheres to the entire part having a large amount of the solid phase, or the liquid phase seeps through the solid phase. In this case, the product has external appearance having a heterogeneous object adhering thereto. In general, in a die-cast product, this corresponds to poor external appearance called “blister” and “seepage”.
Further, in a region where only the liquid phase solidifies, as compared to the remaining region, a large number of compounds each having the liquid phase as a main component solidify and precipitate, and hence ductility is deteriorated. An outer layer portion of the product is subjected to bending deformation more intensely than an inside thereof, and hence a flex crack may start from a region where the liquid phase adheres to the outer layer and solidifies. Thus, there is also a fear of reduction in mechanical strength.